1. Field of the Invention
The present invention relates to a method of a network paging a user equipment for error recovery in a wireless communication system and related communication device, and more particularly, to a method of a network paging a user equipment with proper identities for error recovery in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) for communicating with a plurality of user equipments (UEs) and communicates with a core network (CN) including a mobility management entity (MME), serving gateway, etc for NAS (Non Access Stratum) control.
To receive services, e.g. evolved packet system (EPS) services, the UE needs to register with the network. During a registration procedure, e.g. attach procedure, the UE may first send an “ATTACH REQUEST” message to the core network (e.g. MME). The “ATTACH REQUEST” message includes an international mobile subscriber identity (IMSI), which is stored in a subscriber identity module (SIM) card inside the mobile device, e.g. UE. Because the IMSI uniquely addresses each subscriber, it is seen as critical information from a security point of view and its transmission clearly has to be avoided as much as possible. By spying on and monitoring the IMSI, eavesdropper may track a subscriber's location, movement, and activities. So the network allocates a temporary UE identity, for example a system architecture evolution (SAE) temporary mobile subscriber identity (S-TMSI), to the UE and renews it frequently in order to reduce the use of IMSI.
Besides, if a UE without a valid IMSI (e.g. the UE does not have a SIM card or the SIM card is invalid) would like to make an emergency call, the UE can still initiates a registration procedure for emergency bearer services. First, the UE sends an “ATTACH REQUEST” message to the network. In such case, the “ATTACH REQUEST” message is allowed to include an international mobile equipment identity (IMEI) instead of IMSI. An IMEI is a unique number which shall be allocated to each individual UE in the public land mobile network (PLMN) and shall be unconditionally implemented by the manufacturer. Then, the network also allocates a temporary UE identity, e.g. S-TMSI, to the UE and renews it frequently in order to reduce the use of IMEI.
In some abnormal cases, the network may still only use IMSI to initiate paging for error recovery, and/or informing UEs to register again/re-attach. For one example, due to a network failure, the S-TMSI is not available or becomes invalid, the network may use the registered IMSIs to initiate paging for EPS services with CN domain indicator set to packet switch (PS). For another example, due to a network failure, the tracking area identity (TAI) list is not available, the network may use the registered IMSIs to perform the paging within all tracking areas served by the network, e.g. the MME. In the above examples, only those UEs having IMSI will be paged, and those UEs having no IMSI will not be paged. In other words, UEs using IMEIs to attach to the network will not be paged. Consequently, UEs using IMEIs to attach to the network will not initiate another registration procedure after a network failure until those UEs need services from the network, causing unnecessary delay. However, generally those UEs using IMEIs to register to the network are for emergency purposes. For example, a car accident survivor using a UE without IMSI would like to make an emergency call so that the UE initiates the attach procedure with its IMEI to the network. If a network failure occurs, the network cannot page the UE only having its IMEI. As a result, the emergency call fails due to the network failure in the paging stage. This may put the survivor in great danger.